Known MR methods, such as rapid imaging methods (echo planar imaging, EPI) and in particular spectroscopy applications place high demands on the homogeneity of the basic or polarization field B0. The individual body of each patient deforms the local field.
In order to be able to apply the methods, however, what is known as “in vivo shimming” is carried out in a patient-specific manner, i.e. the MR system is adjusted to the examined person. In this regard, with conventional methods the local B0 field in the examination region is firstly measured, to create what is known as a B0 map, and on the basis of the B0 map, DC offset currents for the three gradient coils and currents for special shim coils of a higher order are then calculated, such that the local field distortions are compensated in the best way possible. After adjusting these currents, an HF resonance frequency for the desired spectral component of the examined tissue (generally protons bound to water) is generally determined in a frequency adjustment.
The fineness with which the local B0 field inhomogeneities are compensated depends primarily on the number and order of the existing (and usable) shim channels. With modern MR systems, the linear terms x, y and z are generally generated via static offset currents of the three gradient coils. Due to the linear field profile along the gradient direction, the term shim channels of the first order is also used.
Furthermore, many modern MR devices have dedicated shim coils here. These shim coils are frequently designed such that the fields generated thereby can be described by spherical harmonics. The shim channels of the second order comprise for instance 5 shim coils which generate fields, the spatial course of which can be described in good approximation by z2 (channel M4/A20), xz (channel M5/A21), yz (channel M5/A21), x2−y2/2 (channel M6/B21), xy (channel M8/B22).
With conventional methods, there is a special user interface for frequency adjusting the shim settings, in order to perform manual optimizations (what is known as IntShim). A constantly repeating measurement of the frequency spectrum can be started in this user interface, wherein a conventional repetition time amounts to approx. 1 s. The frequency spectrum determined in the adjustment volume (adjust volume, AdjVol) is indicated with a sufficiently high spectral resolution.
The user can now optimize the shim state by manually adjusting the shim settings, by always iteratively further improving the shim state. The width of the peaks in the spectrum is typically used as a quality criterion to determine the quality of the shim state, the half value width (full width at half maximum, FWHM) is typically used as a quantitative variable. Adjustment parameters for this procedure are the three piece gradients (linear or 1st order spherical harmonics) and five piece E-shim (square or 2nd order spherical harmonics) or more if channels of the 3rd order or higher are used.